Numerous techniques exist to deliberately trigger avalanches.
A first technique consists in the positioning of an explosive charge by an operator in the slope where the triggering of an avalanche is required. The positioning of the explosive in the slope may be performed in accordance with two methods, namely throwing it from the ground or a helicopter, and sliding the charges. With regard to the charge initiation, this is classically performed using a slow fuse or electrically.
This first technique presents a certain number of risks for the powdermen. Specifically, their interventions necessarily take place during periods of considerably unstable snow accumulation and in dangerous areas. They are therefore exposed to the risk of an avalanche, not only during preparation and realization of the blast, but also during their movements to and from the firing station, in other words, the place for preparing the blast and for igniting the charge. These risks are the main cause of accidents during triggering operations. In the case of using a helicopter, the risks inherent to transporting and initiating explosives in an aircraft must be added, all the more in often delicate aerological conditions.
Remote triggering techniques have been implemented in order to prevent the powdermen from moving in the firing zone. The objective is to move the firing station away from the firing point, in other words, from the position of the charge at the moment it explodes.
A remote triggering device is known under the name of CATEX. This is a conveyor cable mounted on a fixed infrastructure, enabling an explosive to be brought into a predetermined firing zone that is accessible by the conveyor cable.
Such a solution, although reducing the risks for the operator, only allows avalanches to be triggered in zones served by the cable. Moreover, such a technological solution involves the transport and storage of explosives, which calls for stringent safety criteria to be met. Finally, the installation of a long-distance conveyor cable remains very expensive.
Another device is known under the name of GAZEX. This is described in document FR 2 636 729. It comprises a gun with a closed base having an open frontal mouth directed towards the snow accumulation. Moreover, it comprises an oxidizing gas supply circuit and a fuel gas supply circuit, the two gases coming from two distinct sources. The nozzles for filling the gun with these gases are arranged on various zones of the gun and an ignition device is mounted at the rear of the latter. The explosive gas mixture, composed for example of propane and oxygen, is formed in the gun, the explosion being caused by the ignition device. More specifically, the gun takes the general shape of a neck, the explosive gas mixture having a greater density to air and thus accumulating in the lower section of the gun without escaping through the open frontal mouth.
This device, although having proved its effectiveness, must be mounted in a fixed manner in the zone at risk. Consequently, it cannot be transported easily, which requires the mounting of a device on each firing zone.
U.S. Pat. No. 4,873,928 describes a device to generate a shock wave through the explosion of an explosive gas contained in a balloon. The device comprises an expandable balloon, a device for filling the balloon with an explosive mixture of oxygen and hydrogen, and an ignition device designed to trigger the explosion.
Document EP 1 031 008 describes a similar device in which the balloon is simply fixed to a support, the end facing downwards, such that during inflation, the balloon is directed upwards.
A large part of the shock wave generated by the explosion of the balloon is lost and is not transmitted to the snow accumulation, due to the orientation of the explosion.
Such a device does not therefore produce optimum results. Specifically, as the balloon is fixed to a support and facing upwards, the majority of the explosion is directed upwards and laterally since the support acts as an obstacle for the movement of the wave between the balloon and the snow accumulation. In the same manner as previously, this remote triggering device cannot be transported.
Other remote triggering techniques use military weapons. In this way, rocket launchers or mine launchers are used, mainly in Switzerland, or recoilless guns or the LoCAT shell launcher are used in the United States of America.
However, some legislation, and in particular legislation in France, prohibits the storage of primed charges, which renders the use of such devices impossible.
In order to remedy these disadvantages, document WO 2007/096524 proposes a transportable avalanche triggering device, the explosion of which is mainly directed towards the snow accumulation and requiring no transportation or storage of explosives, and whose use conforms with the various national legislations.
It comprises a frame fitted with means of attachment for transporting the device, especially by helicopter with the aid of a cable, the frame comprising, at the top, a storage area for at least one gas container designed to form an explosive mixture and, at the bottom, a device for holding a plurality of elastic balloons, each having an inflation end oriented upwards, the body of each balloon extending in the opposite direction and the balloons being separated from each other. Furthermore, this device comprises means for conveying the explosive mixture to the inflation end of a balloon, an injection nozzle, means for igniting the explosive mixture, means for successively bringing the injection nozzle and the ignition means to the inflation end of each balloon.
Such a device can be transported over various firing zones above the snow accumulation in order to cause an avalanche by the explosion of a balloon located at the bottom of the device. The majority of the explosion is thus towards the snow accumulation.
In addition, such a device does not require the transportation of explosives, as the explosive mixture is created on the site before firing, which, in fact, enables a safe distance to be respected.
Furthermore, as this device is fitted with several balloons, it is possible to perform a series of explosions, which enables either guaranteeing the triggering of an avalanche by repeating a first firing without effect, or triggering several avalanches in different areas, without having to recharge the device.
However, such a device presents the following disadvantages.
The casing that enables the mixture to be contained until the explosion may pose inflation problems or premature explosion.
In addition, such devices, despite using several balloons, require the system to be recharged frequently.
The mechanical systems necessary for the device to function are, furthermore, potentially fragile or the source of problems in certain conditions of use (cold, frost, etc.)